Carrier wave modulator arrangement



June 13, 1939.

c. s, COCKERELL CARRIER WAVE MODULATOR ARRANGEMENT Filed Dec. 12, 1936 iourpur F/LTER F02 R. F.

INVENTOR (.5. (OCKERELL BY ATTORNEY Patented June 13, 1939 UNITED STATESATET FFECE CARRIER WAVE MODULATOR ARRANGEMENT Application December 12,1936, Serial No. 115,487

In Great Britain 6 Claims.

This invention relates to carrier wave modulator arrangements and moreparticularly, though not exclusively, to modulator arrangements capableof being employed to modulate the amplitude of a high frequency carrierin accordance with televison or like signals.

There are many known methods of modulation, but when these methods areemployed to secure modulation of the amplitude of a high frequencycarrier in accordance with television or similar signals, seriouspractical difiiculties are encountered. A modern high fidelitytelevision system may involve modulation frequencies ranging from about25 cycles per second to about 3 megacycles per second, and themodulation voltage normally available is not very great. As will readilybe appreciated such a very wide range of modulation frequencies involvesserious practical difficulties and the main requirements, namely the g0achieving of as nearly as possible rectilinear modulation over the wholefrequency range without requiring the modulation source to supply anyappreciable amount of power, are very difiicult to satisfy. As will beseen later the present invention provides improved modulation systems inwhich these requirements are to a large extent satisfied.

According to this invention there is employed a carrier frequencyamplifier whose gain is automatically controlled in dependence upon theoutput from sad amplifier in such manner that in the absence ofmodulating potentials the output from said amplifier is maintained at apredetermined value, and gain control in dependence upon modulationpotentials is superimposed upon the gain control in dependence uponoutput in such manner that the output carrier frequency amplitude variessubstantially rectilinearly with the modulation input.

The invention is illustrated in the accompanying drawing which shows inblock diagram form a preferred Way of carrying out the said invention.

Referring to the drawing the carrier frequency to be modulated isapplied from a source not shown across an input impedance I of anysuitable nature which is included in the grid circuit of the first tube2 of a carrier frequency amplifier represented by a rectangle 3 andwhich may 5 consist of the said tube only or of any desired number oftubes in cascade. The cathode or cathodes of the tube or tubes in thecarrier frequency amplifier 3 is or are earthed. The output circuit ofthe last tube 4 in the carrier frequency amplifier 3 (this tube will, ofcourse,

December 27, 1935 be the tube 2 if the amplifier 3 contains only onetube) includes an output impedance 5, e. g., a resistance, which isearthed at the end remote from the anode. Carrier frequency voltage setup across impedance 5 is applied through a suitable condenser 6 to theanode 1 of a diode 8 whose cathode 9 is earthed through a cathodeimpedance lii-for example a resistance-4n series with a battery H orsuitable fixed direct current potential source, the negative terminal ofthe said source being connected to earth and the positive terminal beingconnected to that end of the cathode impedance ll remote from thecathode 9.- The diode anode 1 is connected through a suitable filter l2to that end of the input impedance 1 remote from the control grid of thefirst tube 2 of the amplifier 3. Modulating potentials from a source notshown are applied between the diode cathode 9 and earth.

With the above described arrangement, it will be seen that the firsttube 2 of the carrier frequency amplifier 3 will receive gaincontrolling potential from the diode rectifier circuit. The arrangementis such that in the absence of modulating potentials, the voltage builtup across the output impedance 5 is greater than the Voltage between thediode cathode 9 and earth by an amount such as will result in a negativegain controlling potential of sufficient magnitude to reduce the gain ofthe carrier frequency amplifier 3 to rather less than half its maximumvalue. If now, modulating potentials are superimposed between the diodecathode 9 and earth, the gain control potential applied at any instancewill depend upon the modulation input, and by suitably selecting theparameters of the circuit it is possible to obtain a substantiallyrectilinear modulation action up to a high percentage of modulation.

The invention is, of course, not limited to the application of gaincontrolling potential to the grid of only the first tube 2 of thecarrier frequency amplifier, for, if desired, more than one tube of thisamplifier may be controlled as to its gain in the manner described.Again the output energy taken from the amplifier for gain controllingpurposes need not be taken in the manner described; for example, it maybe taken by inductive coupling or even by taking up radiation from anaerial system energized by the transmitter of which the carrierfrequency amplifier forms part. An advantage of taking the said outputenergy in the last described manner, i. e., by radiation from an aerialsystem is that by resorting to this expedient it is possible to correctautomatically for any frequency distortion in the transmitter-includingany due to the aerial. The diode 8 may be replaced by any suitable'typeof detector, e. g., a multi-grid tube,

the output from which is a function of the difference between the peakor mean or root mean square value of the carrier wave 'andtheinstantaneous value of the modulatingpotentials,

the said output being utilized to control the gain of the carrierfrequency amplifier.

I It is found that with modulation 'systems in accordance with thisinvention, satisfactory operation over a very wide band of modulating.fre? quencies may be obtained; rectilinear or closely approximatelyrectilinear modulation may be obtained over substantially the whole of awide band of modulating frequencies; quite deep modulation up to a highpercentage of modulation may be obtained; the modulating potentialsrequired are relatively small; substantially no power is required formodulation, i. e., the source "of modulating potentials is substantiallyunloaded; and all the tube cathodes can be earthed-an important matterfrom the point of view of stray capacity effects when high modulatingfrequencies are in question.

I claim: 7 r r V 1. In a carrier wave'modulation system suitable formodulating the amplitude of a high frequency carrier wave in accordancewith television or like signals, a high frequency amplifier tube havinginput and output electrodes connected in input and output circuitsrespectively,- means for applying carrier' wave energy to' be modulatedto the input circuit of said amplifier tube, a direct current impedanceconnected between the output electrodes of said amplifier tube,a'rectifier tube having an anode and a cathode, a series circuitincluding a resistance coupling the anode and cathode of said rectifiertube in shunt to said impedance whereby wave energy from said amplifieroutput is impressed on said rectifier, a

control circuit connected in shunt to a portion of saidseries circuit,acoupling between said control circuit and an electrode in saidamplifier tube for applying gain controlling potentials to saidelectrode of said amplifier tube, and means for impressing modulatingpotentials on said rectifier tube to control thegain of said amplifiertube in accordance with modulating potentials and potentialscharacteristic of modulated carrier energy supplied to said controlcircuit by said rectifier, the relative values of said first impedanceand said resistance and the impedance of said rectifier tube being suchthat in the absence of modulating potentials the gain of said amplifiertube is adjusted to approximately one-half its imposed modulatingpotentials and rectified'potentials" characteristic of the modulatedcarrier energy isto vary the gain of said amplifier sub-'stantiallyrectilinearly about said adjusted value.

2. An arrangement as recited in claim 1 wherein the control circuit iscoupled to the electrode in said amplifier tube by means of a filter forsuppressing wave energy of a frequency of the order of the frequency ofthe carrier wave energy.

3. An arrangement as recited in claim 1 wherein said electrode in saidamplifier to which said control potentials are impressed is an inputelece trode;

maximum value so that the effect of the super- 4. A wave relaying tubehaving input electrodes 7 output of said relaying'tube, a gain controlcir-' Vcuit connecting said rectifier to an electrode in said relayingtube, and means for impressing modulating potentials on said rectifier.

5-. A system as recited in claim 4 wherein: said first impedance is ofsuch a value relative to the value of said second impedance that thepotential drop produced in said first impedance by the output of saidrelaying tube in the absence of saidmodulating potentials is sufficientto produce a negative gain control potentialin said gain controlcircuit. V

6'. A system as recited in claim f1: wherein said' gain control circuitincludes a filter for energy of a frequency of the order of thefrequency of the wave energy to be modulated.

CHRISTOPHER SYDNEY COCKERELL.

